Generally, marine speed log devices employ sensing transducers having a magnetic coil comprising many turns of fine wire about an iron core or laminated iron core. The coil is periodically energized at a frequency of 50 or 60 Hz, which corresponds to the available power line frequency, which allows the speed log device to become sensitive to signals induced from a field caused by the available AC source power, causing erroneous indications.
Speed log systems have required a different calibration procedure for each five knot interval in velocity. Furthermore, it is often necessary to provide a switch to accommodate a change from fresh to salt water, due to the change in conductivity of the water.
Speed logs have generally provided velocity information in the forward direction alone, making it impossible to determine the total distance travelled. It is frequently required that the speed be indicated in the reverse direction as well to provide accurate navigation and position determination.